A hydrotreater is the refinery unit that uses hydrogen to remove impurities from petroleum products.
In hydrotreating, a petroleum stream is mixed with hydrogen and passed over a catalyst at elevated temperature and pressure. Sulfur is converted largely into hydrogen sulfide, while nitrogen compounds and some other contaminants are also removed or reduced. The treated product can then meet fuel-quality and emissions standards.
Hydrotreaters are used on streams such as naphtha, kerosene, diesel, and gas oils. Their severity depends on the feedstock and the desired product. Refineries often recover sulfur from the resulting hydrogen sulfide in a separate sulfur-recovery process.
Hydrotreater is a broad equipment term, while hydrodesulfurization describes the sulfur-removal reaction specifically. A hydrocracker also uses hydrogen, but it combines hydrogen treatment with catalytic breaking of large hydrocarbon molecules. Those are related technologies, not interchangeable names for the same unit.